1. Field of the Invention
The present invention relates to data networking and, in particular, to a technique for coupling geographically dispersed entities belonging to a virtual local area network (VLAN) via virtual ports.
2. Background Information
A data network is a geographically distributed collection of interconnected communication links and segments for transporting data between nodes, such as computers. The nodes typically transport the data over the network by exchanging discrete frames or packets containing the data in accordance with various pre-defined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP) or the Internetwork Packet eXchange (IPX) protocol. In this context, a protocol consists of a set of rules defining how the nodes interact with each other.
Many types of networks are available, with types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect nodes, such as personal computers and workstations, over dedicated private communication links located in the same general physical location, such as a building or a campus to form a private network. WANs, on the other hand, typically connect large numbers of geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes contained in various networks. WANs often comprise a complex network of intermediate network nodes, such as routers or switches, that are interconnected to form the WAN and are often configured to perform various functions associated with transferring traffic through the WAN.
Some organizations employ virtual LANs (VLANs) in their private networks to “logically” group entities, such as users, servers, and other resources within the organization. A VLAN is a logical group of entities, such as users and servers, which appear to one another as if they are on the same physical LAN segment, even though they may be spread across a large network comprising many different physical segments. A VLAN operates at the data link layer, which is layer-2 (L2) of the Open Systems Interconnect (OSI) reference model.
In some organizations, entities belonging to a VLAN group may be dispersed over a wide geographical area. To interconnect the geographically dispersed entities, an organization may subscribe to a service provider (SP) that provides a WAN to enable communication among the various dispersed entities. Here, the organization may employ one or more routers to interconnect the various dispersed entities to the SP's WAN.
Some SPs employ the Asynchronous Transfer Mode (ATM) protocol to carry large volumes of traffic generated by various organizations through the WAN. Moreover, the SP may employ ATM virtual connections (VCs), wherein each VC carries the traffic for a particular organization's VLAN. By employing VCs in this manner, an SP can ensure that traffic generated on one organization's VLAN does not interfere with traffic generated on another organization's VLAN.
One problem with using VCs to carry VLAN traffic is that the VCs may not appear transparent to various L2 protocols operated over the VLAN. For example, nodes belonging to a VLAN often run the spanning-tree protocol (STP) and periodically generate bridged-protocol data units (BPDUs). The STP treats a physical port on these nodes as a single physical point-to-point data link and consequently sends only one copy of a generated BDPU to a given port. An ATM physical port, however, may be associated with a plurality of VCs that couple various network devices belonging to the VLAN. Since only one BPDU is generated for the ATM port, the STP may not operate properly as there will not be enough BPDUs for transfer over all the VCs associated with the VLAN.
Likewise, in accordance with the STP, a physical port may be placed in a blocked state to avoid loops in a particular VLAN's topology. This may pose a problem with ATM implementations wherein a blocked VC blocks an entire ATM port. For example, if the ATM port is associated with a plurality of VCs and each VC is associated with a different VLAN, blocking a VC to meet the requirements of the STP for a particular VLAN may inadvertently cause traffic on the other VLANs to be blocked as well.
Another problem associated with coupling VLANs via VCs is that in some intermediate nodes a separate control structure may be maintained for each VC. The control structure typically holds information associated with the connection, such as connection status and various statistics. Often, the number of control structures available in an intermediate node is limited due to limited resources available to the node, e.g., a limited amount of memory storage. Consequently, if the number of dispersed entities in a VLAN is quite numerous and requires many VCs, an intermediate node in the network may not have sufficient resources to maintain control structures for all the VCs needed to couple the entities belonging to the VLAN.